Conventional spirit levels which are of simple and inexpensive construction comprise a support body which provides a surface to be placed on an object and a bubble tube provided within the body which is generally elongate and positioned with its longitudinal axis either along the length of the surface or transverse thereto so that a gas bubble within the liquid in the bubble tube can be positioned centrally of the bubble tube when the support surface is horizontal or vertical.
Spirit levels of this type are inexpensive and readily available and widely used. However they have a serious disadvantage in that it is very difficult or impossible to use the spirit level in low levels of illumination when the bubble cannot readily be seen.
Alternative forms of level indicating devices have been proposed to overcome this problem of illumination. Generally these solutions provide complicated electrical sensing of the position of a bubble or other floating object. These solutions have found little acceptance generally in the trade probably because they are expensive and complicated in comparison with the simple bubble tube arrangement which is fully satisfactory except in relation to this problem of illumination.
U.S. Pat. No. 4,484,393 of the present inventor discloses an illuminated spirit level which comprises a bubble tube mounted in a support body with light source shielded to define two light paths directed through the bubble tube to apertures on the outside of the support body. With the bubble positioned centrally, the light is communicated directly from the light source through the bubble tube to the apertures. On movement of the bubble relative to the bubble tube caused by inclination of the spirit level, the bubble intercepts one of the light paths so as to cause refraction directing the light away from the aperture. The different refraction is caused by the difference in shape between the lower surfaces of the bubble and the upper surface of the bubble tube. The curvature of the bubble causes the edge to totally reflect the light path to divert the light.
U.S. Pat. No. 7,350,305 (Molitorisz) discloses a levelling device which again uses the optical interaction between a laser beam and a freely floating air bubble confined in a transparent, liquid-filled vial. When the laser beam is impacted by the air bubble, the curved surface of the air-bubble deflects the beam toward the end of the vial, where a photo-electric device is positioned. The photo-electric device activates a sound emitter such as a buzzer.
These devices use the refraction and reflection generated by a bubble and hence have some problems of accuracy.